SOHO Instruments Research Articles

Searching for g modes

Context. The recent claims of g-mode detection have restarted the search for these potentially extremely important modes. These claims can be reassessed in view of the different data sets available from the SoHO instruments and ground-based instruments. Aims. We produce a new calibration of the GOLF data with a more consistent p-mode amplitude and a more consistent time shift correction compared to the time series used in the past. Methods. The calibration of 22 yr of GOLF data is done with a simpler approach that uses only the predictive radial velocity of the SoHO spacecraft as a reference. Using p modes, we measure and correct the time shift between ground- and space-based instruments and the GOLF instrument. Results. The p-mode velocity calibration is now consistent to within a few percent with other instruments. The remaining time shifts are within ±5 s for 99.8% of the time series.

Blowout jets and impulsive eruptive flares in a bald-patch topology

Context: A subclass of broad EUV and X-ray jets, called blowout jets, have become a topic of research since they could be the link between standard collimated jets and CMEs.} Aim: Our aim is to understand the origin of a series of broad jets, some accompanied by flares and associated with narrow and jet-like CMEs. Methods: We analyze observations of a series of recurrent broad jets observed in AR 10484 on 21-24 October 2003. In particular, one of them occurred simultaneously with an M2.4 flare on 23 October at 02:41 UT (SOLA2003-10-23). Both events were observed by ARIES H-alpha Solar Tower-Telescope, TRACE, SOHO, and RHESSI instruments. The flare was very impulsive and followed by a narrow CME. A local force-free model of AR 10484 is the basis to compute its topology. We find bald patches (BPs) at the flare site. This BP topology is present for at least two days before. Large-scale field lines, associated with the BPs, represent open loops. This is confirmed by a global PFSS model. Following the brightest leading edge of the H-alpha and EUV jet emission, we can temporarily associate it with a narrow CME. Results: Considering their characteristics, the observed broad jets appear to be of the blowout class. As the most plausible scenario, we propose that magnetic reconnection could occur at the BP separatrices forced by the destabilization of a continuously reformed flux rope underlying them. The reconnection process could bring the cool flux-rope material into the reconnected open field lines driving the series of recurrent blowout jets and accompanying CMEs. Conclusions: Based on a model of the coronal field, we compute the AR 10484 topology at the location where flaring and blowout jets occurred from 21 to 24 October 2003. This topology can consistently explain the origin of these events.

On the statistical characteristics of radio-loud and radio-quiet halo coronal mass ejections and their associated flares during solar cycles 23 and 24

We have studied the characteristics of radio-loud (RL) and radio-quiet (RQ) front side halo coronal mass ejections (HCMEs) (angular width 360°) observed between the time period years 1996–2014. RL-HCMEs are associated with type II radio bursts, while RQ-HCMEs are not associated with type II radio bursts. CMEs near the Sun in the interplanetary medium associated with radio bursts also affect the magnetosphere. The type II radio burst data was observed by WIND/WAVES instrument and HCMEs were observed by LASCO/ SOHO instruments. In our study, we have examined the properties of RL-HCMEs and RQ-HCMEs and found that RL-HCMEs follow the solar cycle variation. Our study also shows that the 26% of slow speed HCMEs and 82% of fast speed HCMEs are RL. The average speed of RL-HCMEs and RQ-HCMEs are 1370km/s and 727km/s, respectively. Most of the RQ-HCMEs occur around the solar disc center while most of RL-HCMEs are uniformly distributed across the solar disc. The mean value of acceleration of RL-HCMEs is more than twice that of RQ-HCMEs and mean value of deceleration of RL- HCMEs is very small compare to RQ-HCMEs events. It is also found that RQ-HCMEs events are associated with C- and M-class of SXR flares, while RL-HCMEs events are associated with M and X-class of SXR flares, which indicates that the RQ-HCMEs are less energetic than the RL-HCMEs. We have also discussed the various results obtained in present investigation in view of recent scenario of solar physics.

A time series of filament eruptions observed by three eyes from space: from failed to successful eruptions

We present stereoscopic observations of six sequential eruptions of a filament in the active region NOAA 11045 on 2010 Feb 8, with the advantage of the STEREO twin viewpoints in combination with Earth's viewpoint from SOHO instruments and ground-based telescopes. The last one of the six eruptions is a coronal mass ejection, but the others are not. The flare in this successful one is more intense than in the others. Moreover, the velocity of filament material in the successful one is also the largest among them. Interestingly, all the filament velocities are found to be proportional to the power of their flares. We calculate magnetic field intensity at low altitude, the decay indexes of the external field above the filament, and the asymmetry properties of the overlying fields before and after the failed eruptions and find little difference between them, indicating the same coronal confinement exists for both the failed and successful eruptions. The results suggest that, besides the confinement of the coronal magnetic field, the energy released in the low corona should be another crucial element affecting a failed or successful filament eruption. That is, a coronal mass ejection can only be launched if the energy released exceeds some critical value, given the same initial coronal conditions.

Imaging Observations of Quasi-Periodic Pulsatory Nonthermal Emission in Two-Ribbon Solar Flares

Using RHESSI and some auxiliary observations we examine possible connections between spatial and temporal morphology of the sources of non-thermal hard X-ray (HXR) emission which revealed minute quasi-periodic pulsations (QPPs) during the two-ribbon flares on 2003 May 29 and 2005 January 19. Microwave emission also reveals the same quasi-periodicity. The sources of non-thermal HXR emission are situated mainly inside the footpoints of the flare arcade loops observed by the TRACE and SOHO instruments. At least one of the sources moves systematically both during the QPP-phase and after it in each flare that allows to examine the sources velocities and the energy release rate via the process of magnetic reconnection. The sources move predominantly parallel to the magnetic inversion line or the appropriate flare ribbon during the QPP-phase whereas the movement slightly changes to more perpendicular regime after the QPPs. Each QPP is emitted from its own position. It is also seen that the velocity and the energy release rate don't correlate well with the flux of the HXR emission calculated from the sources. The sources of microwaves and thermal HXRs are situated near the apex of the loop arcade and are not stationary either. Almost all QPPs and some spikes of HXR emission during the post-QPP-phase reveal the soft-hard-soft spectral behavior indicating separate acts of electrons acceleration and injection, rather than modulation of emission flux by some kinds of magnetohydrodynamic (MHD) oscillations of coronal loops. In all likelihood, the flare scenarios based on the successively firing arcade loops are more preferable to interpret the observations, although we can not conclude exactly what mechanism forces these loops to flare up.

A multi-periodic oscillatory event in a solar flare

Aims. Ratios of different significant periods found in the light curves of a solar flare exhibiting quasi-periodic pulsations (QPP) are used to distinguish between the possible physical mechanisms responsible for such periodic behaviour. Methods. Time series data of the flaring event of 2002 July 3, observed via the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters, and the RHESSI satellite, are investigated with the use of the Lomb-Scargle periodogram technique. Images of the event are also recovered via the RHESSI, SOHO, and Nobeyama Radioheliograph instruments. Results. Statistical analysis of the Lomb-Scargle periodogram results indicates three distinct periods above the 99% confidence level in Nobeyama Radioheliograph and Radiopolarimeter data, at 28 s, 18 s, and 12 s, respectively. The two longest of these periods were also observed in the RHESSI data at the same confidence level. Wavelet analysis demonstrated that multiple periods occurred simultaneously without any significant frequency shift over time. Reconstructed images of the event reveal a very compact flare structure unsuitable for spatially resolved analysis. Consideration of the period ratios leads to the conclusion that the cause of this multi-periodic event is likely to be a kink mode periodically triggering magnetic reconnection.

Coronal Rotation at Solar Minimum from UV Observations

UVCS/SOHO observations have been analyzed to reconstruct intensity time series of the O VI 1032 A and H 11216 A spectral lines at different coronal heliolatitudes from 1.5 to 3.0 solar radii from Sun center. Evidence was found for coronal differential rotation that differs significantly from that of the photospheric plasma. The study of the latitudinal variation shows that the UV corona decelerates toward the photospheric rates from the equator up to the poleward boundary 2 of the midlatitude streamers, reaching a peak of 28.16+/-0.20 days around +30 from the equator at 1.5 solar radii, while a less evident peak is observed in the northern hemisphere. This result suggests a real north-south rotational asymmetry as a consequence of different activity and weak coupling between the magnetic fields of the two hemispheres. The study of the radial rotation profiles shows that the corona is rotating almost rigidly with height.

Helium Line Formation and Abundance during a C‐Class Flare

During a coordinated campaign which took place in May 2001, a C-class flare was observed both with SOHO instruments and with the Dunn Solar Telescope of the National Solar Observatory at Sacramento Peak. In two previous papers we have described the observations and discussed some dynamical aspects of the earlier phases of the flare, as well as the helium line formation in the active region prior to the event. Here we extend the analysis of the helium line formation to the later phases of the flare in two different locations of the flaring area. We have devised a new technique, exploiting all available information from various SOHO instruments, to determine the spectral distribution of the photoionizing EUV radiation produced by the corona overlying the two target regions. In order to find semiempirical models matching all of our observables, we analyzed the effect on the calculated helium spectrum both of A(He) (the He abundance) and of the uncertainties in the incident EUV radiation (level and spectral distribution). We found that the abundance has in most cases (but not in all) a larger effect than the coronal back-radiation. The result of our analysis is that, considering the error of the measured lines, and adopting our best estimate for the coronal EUV illumination, the value A(He)=0.075 +/- 0.010 in the chromosphere (for T>6300 K) and transition region yields reasonably good matches for all the observed lines. This value is marginally consistent with the most commonly accepted photospheric value: A(He)=0.085.

Physical parameters of a mid-latitude streamer during the declining phase of the solar cycle

Context. Investigating the physical properties of solar coronal streamers is important for understanding their role in the global magnetic structure of the extended solar atmosphere, as well as in the generation of the slow solar wind. Aims. We hope to contribute as completely as possible to the ongoing SOHO instruments campaign devoted to the study of the physical characteristics of coronal streamers at various heliocentric distances. Methods. We analyzed ultraviolet H i Lyα and O vi resonance doublet lines observed by UVCS/SOHO in a narrow, mid-latitude streamer structure along different lines of sight during a week in May 2004 and made nearly simultaneous white-light polarized brightness measurements from the LASCO/SOHO C2 coronagraph. Results. Electron densities and temperatures, H i and O vi kinetic temperatures, and outflow velocities were derived from the line

Velocity Profiles in the Solar Corona from Multi‐Instrument Observations

We present a method to derive outflow velocities in the solar corona using different data sets, including solar wind mass flux coming from the SWAN SOHO instrument, electron density values from LASCO-C2, and interplanetary solar wind velocities derived from ground-based interplanetary scintillation observations (IPS). In a first step, we combine the LASCO electron densities at 6 R☉ and the IPS velocities and compare the product to the SWAN mass fluxes. It is found that this product represents the actual mass flux at 6 R☉ for the fast wind, but not for the slow wind. In regions dominated by the slow wind, the fluxes derived from SWAN are systematically smaller. This is interpreted as proof that the fast solar wind has reached its terminal velocity at ~6 R☉ and expands with constant velocity beyond this distance. On the contrary, the slow solar wind has reached only half of its terminal value and is thus accelerated farther out. In a second step, we combine the LASCO-C2 density profiles and the SWAN flux data to derive velocity profiles in the corona between 2.5 and 6 R☉. Such profiles can be used to test models of the acceleration mechanism of the fast solar wind.

Manifestations of the ray structure of the coronal streamer belt in the form of sharp peaks of the solar wind plasma density in the Earth’s orbit

White corona images are analyzed based on the calibrated data of the LASCO-C2/SOHO instrument (processing level 1) and the solar wind (SW) parameters with hourly and minute resolutions on the Wind spacecraft. The quasistationary events, excluding coronal mass ejections and their manifestations in SW are studied. It has been indicated that the angular dimensions and relative variations in the density of the streamer belt segments remain almost unchanged over the entire distance from the Sun to the Earth’s orbit. In the Earth’s orbit, the ray structure of the belt streamer manifests itself as sharp (with steep fronts lasting several minutes and less) peaks (of several hours in duration) of the solar wind plasma density with maximal values N max > 10 cm−3.

Helium Line Formation and Abundance in a Solar Active Region

An observing campaign (SOHO JOP 139), coordinated between ground based and SOHO instruments, has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines. The chromospheric lines CaII K, Halpha and Na D as well as HeI 10830, 5876, 584 and HeII 304 AA lines have been observed.These simultaneous observations allow us to build semi-empirical models of the chromosphere and low transition region of an active region, taking into account the estimated total number of photoionizing photons impinging on the target active region and their spectral distribution. We obtained a model that matches very well all the observed line profiles, using a standard value for the He abundance ([He]=0.1) and a modified distribution of microturbulence. For this model we study the influence of the coronal radiation on the computed helium lines. We find that, even in an active region, the incident coronal radiation has a limited effect on the UV He lines, while it results of fundamental importance for the 5876 and 10830 lines. Finally we build two more models assuming values of He abundance [He]= 0.07 and 1.5, only in the region where temperatures are larger than 1.* 10^4 K. This region, between the chromosphere and transition region, has been indicated as a good candidate for processes that might be responsible for strong variations of [He]. The set of our observables can still be well reproduced in both cases changing the atmospheric structure mainly in the low transition region. This implies that,to choose between different values of [He], it is necessary to constrain the transition region with different observables, independent on the He lines.

Coordinating with SOHO

I describe how to maximise the chances of achieving a successful coordinated observation campaign together with SOHO instruments (and TRACE). The ground rules, common pitfalls, useful resources and some essential “tricks of the trade” are covered. The most important hints are: start early, make contact with the instrument teams and the SOCs ( soc@soc.nascom.nasa.gov), ask whenever in doubt, check the calendar, and be specific.

Ray Structure of the Coronal Streamer Belt and Its Manifestation as Sharp Large Peaks of Solar Wind Plasma Density at the Earth's Orbit

The white-light corona calibrated data with processing level L1 from the LASCO-C2/SOHO instrument, and data from the Wind spacecraft with one-hour and one-minute time resolution on quasi-stationary slow (v between 300-450 km/s at the Earth's orbit) the Solar Wind (SW) parameters in the absence of sporadic SW streams are examined. Within distances from the Sun's center less than R in the range of 20-30 Rs,(Rs, the solar radius), slow wind is known as the streamer belt, and at larger distances it is called the Heliospheric Plasma Sheet (HPS). It is shown that the streamer belt comprises a sequence of pairs of rays. In general, ray brightnesses in each pair can differ, and the magnetic field is oppositely directed in them. The neutral line of the radial magnetic field of the Sun runs along the belt between the rays of each of the pairs.The area in which the streamer belt intersects the ecliptic plane and which lies at the central meridian, will be recorded at the earth's orbit with a time delay of 5-6 days, in the form of one or several peaks with Nmax ＞ 10 cm-3. Furthermore, the simplest density profile of the portion of the HCS has the form of two peaks of a different or identical amplitude . The such a profile is observed in cases where the angle of intersection of the streamer belt with the ecliptic plane near the Sun is sufficiently large, i.e. close to 90°. The two-ray structure of the cross-section of the streamer-belt moves from the Sun to the Earth, it retains not only the angular size of the peaks but also the relative density variations, and the position of the neutral line(sector boundary) in between. At the Earth's orbit the ray structure of the streamer belt provides the source for sharp (i.e. with steep fronts of a duration of a few minutes or shorter) solar wind plasma density peaks (of a duration of several hours) with maximum values Nmax ＞ 10 cm-3.

Solar radio observations and development of coronal mass ejections

We present in this study a synthesis of results on the initiation and development of CMEs using coordinated observations from SOHO and ground-based instruments. The emphasis is placed on radio spectral and imaging data. One important goal of this study is the identification of radio signatures of CMEs providing information below one solar radius above the limb. CMEs can be accompanied by radio emission from thermal or non-thermal origin. Multifrequency radio imaging observations allow us to observe the initial steps of CMEs and to get information on the nature of the triggering mechanisms, on the CME lateral progression above the solar limb as well as above the solar disk. They also reveal the dynamical behavior of CMEs or of the eruptive filament cavity in the low and middle corona and make the link with coronagraphic data. Systematic radio surveys showing the space-time evolution of the emitting sources, like the examples presented in this study, are important tools for synoptic studies.

Telescopic and Microscopic views of the magnetosphere: Multispacecraft observations

The magnetospheric research community has long sought the capability to view the Sun-Earth system in a global way and to probe concurrently the microphysical details of key physical regions. This objective has now been substantially realized with the combination of the CLUSTER and IMAGE missions. With the additional use of SOHO, ACE, FAST, SAMPEX, POLAR, and geostationary orbit spacecraft, there is a remarkable ability to apply both telescopic and microscopic principles. As an example, a bright active region on the Sun gave rise on 29 March 2001 to a fast halo coronal mass ejection (CME) event observed by SOHO instruments. Subsequently on 31 March, a strong interplanetary shock wave ahead of a magnetic cloud (probably arising from the earlier CME) passed the ACE spacecraft and hit the Earth’s magnetosphere. This driver compressed the subsolar magnetopause to ≤ 4 RE geocentric distance and initiated a powerful geomagnetic storm (minimum Dst ~ −360 nT). The CLUSTER set of four spacecraft were located in the midnight sector of the magnetosphere near perigee (r~4 RE) at ~0635 UT and observed a dispersionless injection of energetic (E ≥ 20 keV) electrons in association with a large magnetospheric substorm expansion phase onset (AE ~1200 nT). Concurrent to these in situ observations, the IMAGE spacecraft was returning a sequence of global Energetic Neutral Atom (ENA) images from the medium-energy (MENA) and high-energy (HENA) sensor systems. These data showed a very prominent injection of substorm ions in the premidnight (and postdusk) sector of the inner magnetosphere. This event is consistent with a substorm onset that pushed the substorm ‘injection boundary’ far inside of geostationary orbit and far toward the dusk sector. In another event on August 27, 2001, the IMAGE-CLUSTER combination provided evidence that magnetic reconnection began in the mid-tail plasma sheet some 7 min prior to auroral onset and brightening. Alternative interpretations may also be possible even with the multis-pacecraft data available. Notwithstanding, these data gave an unprecedented view both telescopically and microscopically of a magnetospheric substorm onset and help establish key process timing in the magnetosphere. Such available events reveal the power of multispacecraft observations.

The solar influences data analysis centre

Since 1981, the Royal Observatory of Belgium has operated the Sunspot Index Data Centre, the World Data Centre for the Sunspot Index. Recently, the Space Weather Forecast Centre of Paris–Meudon was transferred and added to the activities of the SIDC. Moreover, a complete archive of all images of the SOHO instrument EIT has become available at the SIDC. Given all these extensions, the new style SIDC has become a ‘Solar Influences Data Centre’ that analyses solar activity and provides services on three different time scales: 1. Fast warnings and real time monitoring. As the Regional Warning Centre (RWC) for Western Europe of the International Space Environment Service (ISES), the SIDC collects and redistributes solar, geomagnetic, and ionospheric data in Western Europe. Short-term predictions (3 days) and alerts are produced on a daily basis. 2. Forecasts and middle term analysis. The SIDC takes care of the calculation of a sunspot index, called the International Sunspot Number. We compute and broadcast the daily, monthly, yearly international sunspot numbers, with middle range predictions (up to 12 months). 3. Post-event analysis and long-term solar cycle analysis. Since the launch of SOHO, EIT offers a global view of the EUV corona over the whole rising phase of the solar activity cycle. Such a long-duration data series is unprecedented and allows the study of the evolution over the solar cycle of objects classes such as active regions, coronal holes, coronal mass ejections or flares.

Variations of solar spectral irradiance from near UV to the infrared—measurements and results

Solar spectral irradiance variations are known to exhibit a strong wavelength dependence with the amount of variability increasing towards shorter wavelengths. The bulk of solar radiation is emitted at visible and infrared wavelengths. Thus, the spectral radiation length of 300 nm accounts for 99% of the total solar radiative output. Deposited in the Earth's troposphere and biosphere, this part of the solar irradiance spectrum determines direct solar radiative forcing and is therefore of particular interest for climate studies. First, measurements of solar irradiance and irradiance variability from near UV to the IR are reviewed with particular emphasis on the results obtained from the Variability of Irradiance and Gravity Oscillations (VIRGO) on SOHO and Solar Spectrum Measurement (SOLSPEC) instruments. In the second part a model is presented which describes solar spectral irradiance variations in terms of the changing distribution of solar surface magnetic features.

Sunspot Plumes and Flow Channels

It is well known that sunspots are dark. This statement is not correct in the sunspot atmosphere between the chromosphere and the corona, where sunspots often are brighter than their surroundings. The brightest feature in the sunspot transition region is called a sunspot plume. Not all sunspots contain a plume. We find that 20 out of 21 sunspots show a plume when one magnetic polarity dominates the sunspot region out to a distance of 50 ′′ from the sunspot. Most sunspots show downflows that exceed 25 km s−1 in the sunspot plumes at temperatures close to 250 000 K. This downflow is not maintained by inflow from the corona, but by gas at transition region temperatures, streaming in flow channels from locations well outside the sunspot. We suggest that this inflow is a necessary requirement for the sunspot plume to occur and present a working hypothesis for the origin of sunspot plumes. This paper is the first thorough spectral analysis of sunspot plumes. It is based on simultaneous observations of ten or six EUV emission lines in 42 sunspot regions with the Coronal Diagnostic Spectrometer – CDS on the Solar and Heliospheric Observatory – SOHO. The line profiles are studied in detail with another SOHO instrument, the Solar Ultraviolet Measurements of Emitted Radiation – SUMER.

SOHO and radio observations of a CME shock wave

A 1200 km s−1 Coronal Mass Ejection was observed with the SOHO instruments EIT, LASCO and UVCS on June 11, 1998. Simultaneously, Type II radio bursts were observed with the WAVES experiment aboard the Wind spacecraft at 4 MHz and by ground‐based instruments at metric wavelengths. The density in the shock wave implied by the higher frequency is close to that inferred from the SOHO/UVCS experiment. The drift rates of the Type II radio bursts suggest shock speeds lower than the speed derived from SOHO observations. The SOHO/UVCS spectrum shows enhanced emission in lines of O5+ and Si11+, consistent with modest compression in an MHD shock.